The motor has new pistons, rings and wrist pins. The Babbitt is good on all the rods. The knock goes away when the spark is retarded and it also goes away when you short out #3 plug. The mains have not been touched but the knock is not a heavy one that would suggest a main bearing. Any ideas?
Could be several things, you say new pistons, was the block bored?
good babbitt, was the clearances set correctly,did they go back in the same cyl's the came out of and facing the same way?
rods checked if straight?
Do you notice the knock at idle, or when driving? Is it worse when you are pulling hard such as gaining speed or going uphill? Does it lessen as the engine warms up? Is it worst when you are going along about 25 MPH and just slightly back off the throttle?
All of the above answers could be clues as to what is knocking.
Norm
piston slap?
Block was not bored, clearances correctly set, rods went back on the same journals and the rods were checked for straightness. Knock is audible at high idle but quiets down at idle speed and seems to stay the same as the engine warms up. The knock goes away as the rpms increase after a point but return as the throttle is backed off. I suspected piston slap as I did not check the bores and replaced standard cast iron pistons with new aluminum pistons. I also thought it might be the rear cam bearing but the fact that the noise goes away if you short out #3 plug made me think otherwise.
From your description it sounds like it is a Rod bearing knock. Try taking off the inspection plate and remove the cap from #3 connecting rod. Most rods have been marked on the cam side of the rod with the number. Either a number 3 or 3 dots. The cap and the rod should have the mark on the same side. Go to the auto parts store and buy some plastigauge. Use the type made to measure 3 thousandths or less. Cut a strip of it and place on the cap parallel to the crankshaft and tighten down the cap. This will crush the plastigauge and the width of the strip after it is crushed will determine the clearance. The package the plastigauge has a scale which you can measure to find the clearance. It should be no more than 2 thousandths and no less than 1.5 thousandths that is .0015-.002. If it is over .002 remove a lamentation of shim stock. Re-measure. If it measured within tolerance before you did any adjustments, your knock is somewhere else.
Piston slap usually is worse when you start a cold engine and gets less as it warms up. A main bearing is worse under power and will get less as you retard the spark. A rod is worse when you slack off the gas.
Norm
I'm with Norman.......I'm more than with him except for the plastigauge part as removing shims will be an experiment anyway.
Sounds like a timing gear is worn. Scott
If you remove shim layers only do one side at a time. Meaning remove one layer on one side, try the plastigauge, if too loose remove a layer from the other side, etc till you get it where you want it to be.
An even better way to check then with plastigauge is use a strip of household aluminum foil. Cut the strip about 1 inch wide by about 3 inches long. Fold to make 3 layers (about 1 inch square) This will be about .001 to .0015 thick.
Place the square in the cap and bolt back in place and re-tighten the nuts. If you can turn the crank easily you need to remove shim(s). When you have it set you will be able to turn the crank but it will be snug with the aluminum foil place. If you cant turn the crank then it's too tight.
Val, I think you are on the right track with thinking that it's piston slap.
Standard newspaper also works well. It's about .0015 thick.
Noel
I will try the rod clearance again but I removed some shim on #3 rod when the pistons were changed and checked it with plastigauge. The clearance was between .001 and .0015 after removing some shim. Thanks to all for the insight. Unfortunately, before I started working on the motor it was making so much noise that I can't tell if this knock was there from the start or developed after I worked on it. The wrist pins were so loose that I assumed the noise was coming from them and that is why the pistons, rings and wrist pins were replaced. As soon as I have some time I will go after the source of the knock again and report back. I just hope I don't have to pull the motor to find it.
I can't imagine replacing pistons without any kind of bore measurement. You likely have a lot of taper in one or more cylinders, resulting in piston slap.
Val Soupios:
Did you hone he wrist pin holes in the pistons. I find that the pistons I get in ALWAYS, ALWAYS, have at least one pin that is too tight and that will cause a strong piston slap. It will sound just like a rod knocking and can crack your piston. I have found that I find at least one and some time as many as three wrist pins that are way too tight.
Before you ever install the con rod take the piston and slap it in the palm of your hand. If the wrist pin does not move IT IS TOO TIGHT.
Another method to check you wrist pins, that I don't like as well, and don't do is to hold the piston horizontal and see if the weight of the big end of the con rod will move the con rod down.
Find someone with a fixture to hone your piston if the wrist pin is too tight.
I did the big end drop method on all four pins and they were loose enough so I don't think that is the problem. Assuming it's not something else, I think Royce is right. I should have checked the bores.